This is happening more and more to my iMac 27". There is a pattern of dead pixels (or something) as seen in this image.

That is a screenshot of a white background Photoshop CS5 document zoom level 33% at 300DPI, RGB color mode.

Zoomed in:

Over an image:

This is extremely annoying and started happening a couple weeks ago. I've deleted my Preferences file and it still happens. Anyone have any idea what this is? It only happens at certain zoom levels. If I zoom in a lot or stay very zoomed out, it is not there. This pattern is the same across multiple documents and it's driving me crazy! It looks like confetti over my documents and it is making image editing very difficult.